Where does the copper pipe come from?
The cast iron section needs to be replaced. Depending on where you live, you may not have a choice but to replace it with CI. Otherwise, it's generally easier and less expensive to replace with PVC (or ABS, if that's what's used in your area). As to the copper, yes, it could be a drain, but based on the size, I'd question what it may be supporting. Whatever it is, going into that larger section and then necking down to a smaller section means that instead of any waste just flowing through, that section will always have water and waste in it (at least based on what I think are the installed angle). It's generally never allowed to go from a bigger pipe and funnel it into a smaller one in waste plumbing - you can only stay the same or get bigger.